Does Obama benefit from an active primary campaign?

I decided to look at something relatively basic tonight that I should have looked at a long time ago. Do the Democratic candidates fare better in the polls in the states in which there has been an active primary campaign?

It turns out that the answer appears to be yes for one of the candidates.

Consider the following:

In Ohio, where there has been a primary campaign, Barack Obama leads McCain by 2.6 points in our weighted average of polls. In Pennsylvania, where there hasn’t yet been a campaign, Obama trails McCain by 1.3 points.

In Washington, where there has been a primary campaign, Obama leads McCain by 9.7 points. In Oregon, where there hasn’t been a campaign, Obama leads McCain by 6.3 points.

In North Dakota, where there has been a campaign, Obama led McCain by 4 points in the only poll. In South Dakota, where there hasn’t yet been a campaign, he trails McCain by an average of 6.6 points.

In Tennessee, where there has been a campaign, Obama trails McCain by 15.4 points. In Kentucky, where there hasn’t been a campaign, he trails McCain by 23.3 points.

These numbers — apples-to-apples comparisons of apparently similar states — seem to lead to the conclusion that Obama might benefit from having had the opportunity to campaign in a state, perhaps on the order of about 5 points in his general election polls. And if we plop a ‘campaign’ dummy variable into our regression analysis — which is set to ‘1’ in every state that has voted in a primary or caucus so far, and ‘0’ in the ten states (including Florida and Michigan) that haven’t, it turns out to be highly statistically significant:

OK — so I know that you didn’t come here to read regression output. But what that is saying is that Obama is polling about 5.7 points better in states that have participated in the primary process so far, all else being equal. And it’s saying that this finding is highly unlikely (around 1000-to-1 against) to be the result of chance alone. This is a robust finding too. If you remove other variables that might be related to the presence of a campaign — the fundraising numbers, for instance — the campaign variable continues to show up at about the same level of significance.

This pattern does not show up for Clinton. Her campaign variable is not statistically significant.

But this has been one of the big themes with Barack Obama this year: the more voters get a chance to know him, the more they seem to like him. That’s how he improved his standing in essentially every state that has voted so far as time wore on; check out the pollster.com graphs for more detail.

Now, there are two ways to interpret this data. Number one is that the states where Obama campaigned are still basking in his afterglow, and will eventually come back down to earth. That’s the pessimistic interpretation. The optimistic interpretation is that these improvements in his poll standing are permanent, and that he can therefore expect to gain in general election polls in states like Pennsylvania, Indiana and North Carolina as the campaign wears on. If the optimistic interpretation is true, that means that both Florida and North Carolina should be highly competitive in the fall, and maybe Indiana too, and that that Obama can expect to move back ahead of McCain in the Pennsylvania polling averages.

Nate Silver is the founder and editor in chief of FiveThirtyEight. @natesilver538